A study of methanol and silicon monoxide production through episodic explosions of grain mantles in the Central Molecular Zone
2017
Methanol (CH3OH) is found to be abundant and widespread towards the Central Molecular
Zone, the inner few hundred parsecs of our Galaxy. Its origin is, however, not fully understood.
It was proposed that the high cosmic ray ionization rate in this region could lead to a more
efficient non-thermal desorption of this species formed on grain surfaces, but it would also
mean that this species is destroyed in a relatively short time-scale. In a first step, we run
chemical models with a high cosmic ray ionization rate and find that this scenario can only
reproduce the lowest abundances of methanol derived in this region (∼10−9–10−8). In a
second step, we investigate another scenario based on episodic explosions of grain mantles.
We find a good agreement between the predicted abundances of methanol and the observations.
We find that the dominant route for the formation of methanol is through hydrogenation of
CO on the grains, followed by the desorption due to the grain mantle explosion. The cyclic
aspect of this model can explain the widespread presence of methanol without requiring any
additional mechanism. We also model silicon monoxide (SiO), another species detected in
several molecular clouds of the Galactic Centre. An agreement is found with observations for
a high depletion of Si (Si/H ∼ 10−8) with respect to the solar abundance.
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